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<div class="section" id="placeholders">
<h1><a class="toc-backref" href="./lambda-details.html#id55" name="placeholders">Placeholders</a></h1>
<p>The definition of &quot;placeholder&quot; may surprise you:</p>
<div class="admonition-definition admonition">
<p class="admonition-title first">Definition</p>
<p>A <strong>placeholder</strong> is a metafunction class of the form <tt class="literal"><span class="pre">mpl::arg&lt;X&gt;</span></tt>.</p>
</div>
<div class="section" id="implementation">
<h2><a name="implementation">Implementation</a></h2>
<p>The convenient names <tt class="literal"><span class="pre">_1</span></tt>, <tt class="literal"><span class="pre">_2</span></tt>,... <tt class="literal"><span class="pre">_5</span></tt> are actually
<tt class="literal"><span class="pre">typedef</span></tt>s for specializations of <tt class="literal"><span class="pre">mpl::arg</span></tt> that simply
select the <em>N</em>th argument for any <em>N</em>. <a class="footnote-reference" href="#config" id="id12" name="id12">[6]</a>  The
implementation of placeholders looks something like this:</p>
<table class="footnote" frame="void" id="config" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id12" name="config">[6]</a></td><td>MPL provides five placeholders by default.  See
the Configuration Macros section of <a class="reference" href="./reference-manual.html">the MPL reference manual</a> for a
description of how to change the number of placeholders
provided.</td></tr>
</tbody>
</table>
<pre class="literal-block">
namespace boost { namespace mpl { namespace placeholders {

template &lt;int N&gt; struct arg; // forward declarations
struct void_;

template &lt;&gt;
struct arg&lt;<strong>1</strong>&gt;
{
    template &lt;
      class <strong>A1</strong>, class A2 = void_, ... class A<em>m</em> = void_&gt;
    struct apply
    {
        typedef <strong>A1</strong> type; // return the first argument
    };
};
typedef <strong>arg&lt;1&gt; _1</strong>;

template &lt;&gt;
struct arg&lt;<strong>2</strong>&gt;
{
    template &lt;
      class A1, class <strong>A2</strong>, class A3 = void_, ...class A<em>m</em> = void_
    &gt;
    struct apply
    {
        typedef <strong>A2</strong> type; // return the second argument
    };
};
typedef <strong>arg&lt;2&gt; _2</strong>;

<em>more specializations and typedefs...</em>

}}}
</pre>
<!-- @example.replace('...','') -->
<p>Remember that invoking a metafunction class is the same as invoking
its nested <tt class="literal"><span class="pre">apply</span></tt> metafunction.  When a placeholder in a lambda
expression is evaluated, it is invoked on the expression's actual
arguments, returning just one of them.  The results are then
substituted back into the lambda expression and the evaluation
process continues.</p>
</div>
<div class="section" id="the-unnamed-placeholder">
<h2><a name="the-unnamed-placeholder">The Unnamed Placeholder</a></h2>
<p>There's one special placeholder, known as the <strong>unnamed
placeholder</strong>, that we haven't yet defined:</p>
<pre class="literal-block">
namespace boost { namespace mpl { namespace placeholders {

<strong>typedef arg&lt;-1&gt; _;</strong> // the unnamed placeholder

}}}
</pre>
<!-- @ stack[-2].prepend('namespace shield {') 
example.append('}') # so we don't conflict with the prefix
compile('all') -->
<p>The details of its implementation aren't important; all you really
need to know about the unnamed placeholder is that it gets special
treatment.  When a lambda expression is being transformed into a
metafunction class by <tt class="literal"><span class="pre">mpl::lambda</span></tt>,</p>
<blockquote>
the <em>n</em>th appearance of the unnamed placeholder <em>in a given
template specialization</em> is replaced with <tt class="literal"><span class="pre">_</span></tt><em>n</em>.</blockquote>
<p>So, for example, every row of Table 1.1
below contains two equivalent lambda expressions.</p>
<table border="1" class="table">
<caption>Unnamed Placeholder Semantics</caption>
<colgroup>
<col width="48%" />
<col width="52%" />
</colgroup>
<tbody valign="top">
<tr><td><pre class="first last literal-block">
mpl::plus&lt;_,_&gt;
</pre>
</td>
<td><pre class="first last literal-block">
mpl::plus&lt;_1,_2&gt;
</pre>
</td>
</tr>
<tr><td><pre class="first last literal-block">
boost::is_same&lt;
    _
  , boost::add_pointer&lt;_&gt;
&gt;
</pre>
</td>
<td><pre class="first last literal-block">
boost::is_same&lt;
    _1
  , boost::add_pointer&lt;_1&gt;
&gt;
</pre>
</td>
</tr>
<tr><td><pre class="first last literal-block">
mpl::multiplies&lt;
   mpl::plus&lt;_,_&gt;
 , mpl::minus&lt;_,_&gt;
&gt;
</pre>
</td>
<td><pre class="first last literal-block">
mpl::multiplies&lt;
   mpl::plus&lt;_1,_2&gt;
 , mpl::minus&lt;_1,_2&gt;
&gt;
</pre>
</td>
</tr>
</tbody>
</table>
<!-- @ for n in range(len(stack)):
    stack[n].wrap('typedef ', 'type%d;' % n)
compile('all') -->
<p>Especially when used in simple lambda expressions, the unnamed
placeholder often eliminates just enough syntactic &quot;noise&quot; to
significantly improve readability.</p>
</div>
</div>

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